Changes in residual limb volume over time are an important
challenge for people who use prosthetic limbs. Residual limb volume
changes, both diurnal and long-term, can cause the fit of the prosthetic
socket to change. If the residual limb reduces in volume, then the
socket becomes loose and bony prominences are subjected to greater
stress, potentially causing pain and increasing risk of injury. If the
residual limb increases in volume, then socket pressures on the limb
will increase and blood flow can be restricted, limiting nutrient
delivery and causing a buildup of cell waste in the tissues. Thus, a
technology that controlled limb volume and kept it stable might overcome
these problems and reduce pain and injury in people using prosthetic
limbs.

Elevated vacuum, also termed vacuum assist, was introduced in the
prosthetics industry approximately 15 years ago with an aim of reducing
volume loss in the residual limb over time [1]. An elevated vacuum
system attaches to a prosthesis and draws a vacuum at the distal end of
the socket. Particularly during swing phase, the vacuum pulls residual
limb soft tissues outward, thereby lowering pressure within the
interstitial fluid inside the residual limb. The reduction in
interstitial fluid pressure increases the arterial to interstitial
pressure gradient while decreasing the venous to interstitial pressure
gradient. The result may then be an increase in the amount of fluid
leaving the arterial vasculature into the interstitial space and a
decrease in the amount of fluid moving from the interstitial space into
the venous system [2]. The net result is then a reduction in fluid loss
out of the residual limb, i.e., less residual limb volume reduction.
Thus, elevated vacuum may serve to reduce limb fluid volume loss. This
effect would be beneficial to people who would otherwise lose limb fluid
volume over the day.

Studies have been conducted to investigate transtibial amputee limb
volume changes using elevated vacuum systems [3-5]. Board et al.
reported limb volume differences over the course of a 30 min walking
session by measuring predonning and postdoffing limb volumes [3]. They
compared results using sockets with elevated vacuum versus sockets
without vacuum (suction). All 10 subjects tested had their amputation as
a result of traumatic injury or were congenital amputees. The tests for
each subject were conducted on the same day, and the same socket was
used for both tests. Results showed that all residual limbs underwent
volume reduction for the novacuum (suction) condition. The mean volume
reduction was 6.5 percent. Nine of ten subjects underwent volume
enlargement for the with-vacuum condition. The mean volume increase was
3.7 percent. Goswami et al. extended from these studies to investigate
residual limb volume changes after 30 min of walking with three
differently sized sockets: 4 percent (by volume) undersized, optimally
sized, and 4 percent oversized [4]. Though who defined "optimally
sized" and how are unclear, the undersized and oversized designs
were achieved with computer-aided manufacturing methods and were
estimated to be accurate to within [+ or -]2 percent volume. Eleven
subjects were enrolled, though only seven subjects were tested with all
three sockets. Use of undersized sockets resulted in a mean volume loss
of 1.8 percent from the day's volume baseline. Use of optimally
sized sockets resulted in a mean volume gain of 7.0 percent from the
day's baseline. Use of oversized sockets resulted in a mean volume
gain of 12.9 percent from the day's baseline. These results suggest
that elevated vacuum can increase limb volume if the socket is
sufficiently large. In a single-subject study, Gerschutz et al. showed
that percent absolute volume changes in the residual limb over a 10 min
period after walking and doffing the socket were less after an elevated
vacuum socket was worn than after a suction socket was worn [5].

Though Board et al., Goswami et al., and Gerschutz et al. reported
the first data in the literature testing elevated vacuum sockets [3-5],
the researchers did not measure in-socket limb volumes. Board et al. and
Goswami et al. casted each subject's residual limb in alginate
before donning and after doffing, then filled the casts with water to
determine limb volume [3-4]. They determined volume differences over a
session by comparing the predonning with the postdoffing alginate cast
volumes [3] or the day's baseline volume with that measured
postdoffing [4]. Gerschutz et al. measured pre- and postdoffing volumes
by using a laser scanner on the subject's residual limb while the
subject wore an elastomeric liner [5].

The purpose of our study was to extend from the previous
out-of-socket investigations to determine whether an in-socket volume
measurement technique produced results consistent with previous
findings. A series of case studies are presented. Using bioimpedance
analysis, we measured extracellular fluid volume changes on individuals
with transtibial amputation while they ambulated with either elevated
vacuum sockets, suction sockets, or sockets with lock-and-pin
suspension. For bioimpedance results in the present investigation to be
consistent with previous findings, we would expect that limb fluid
volume would be maintained or would increase during walking when
elevated vacuum was used. We would also expect that for a high vacuum
pressure compared with a low vacuum pressure or compared with a
lock-and-pin suspension socket, limb fluid volume would decrease less
(or increase more) during walking, cyclic (within-step) fluid volume
changes during walking would reduce (because of improved suspension),
and limb fluid volume would decrease less (or increase more) over 30 min
test sessions.

METHODS

Subjects

Seven subjects with unilateral transtibial amputation participated
in this study. All had their amputation for at least 1 yr and could walk
comfortably without assistive devices for at least a 5 min period. All
were fitted by certified prosthetists trained in elevated vacuum socket
design. During data collection sessions, all socket fits were deemed by
the research prosthetist (coauthor) to be acceptable for regular use.

Bioimpedance Analysis

Bioimpedance analysis is a technique commonly used to assess body
composition/body fat [6-11] as well as fluid imbalance in hemodialysis
patients [12-15]. Bioimpedance takes advantage of the difference in
response of different biological structures to electrical current.
Current will readily pass through cell-free biological fluid at all
frequencies between approximately 5 kHz and 1 MHz. However, it will
easily penetrate cell membranes only at high frequencies, not at low
frequencies. Thus, by applying current across a range of frequencies to
a limb segment, measuring voltage potential in a section of the limb
within the electric field, and then using a computational model to
process the data [16], one can determine extracellular and intracellular
fluid volumes [17].

Volume Measurement

Residual limb extracellular fluid volume was measured with use of
bioimpedance analysis. A low current was applied between two outer pair
electrodes on the residual limb while voltage was measured between two
inner pair electrodes (Figure 1). The electrical current (<700 |J,A)
was applied at 50 frequencies between 5 kHz and 1 MHz with a commercial
device (Hydra 4200, XiTRON Technologies; San Diego, California). One set
of frequencies was sampled each second. Because cell membranes have a
high capacitance, at low frequencies, current passes primarily through
extracellular tissues, while at high frequencies, it passes through both
extracellular and intracellular tissues. Bone and adipose tissue are
minimally conductive, while skin and muscle are highly conductive. Thus,
bioimpedance data reflect primarily fluid changes within skin and muscle
[18]. The collected data, amplitude and phase change at 50 frequencies,
were then fitted to an electrical analog model [17] (Cole model) to
determine extracellular fluid resistance. With use of the Cole model,
tissue was modeled as an extracellular resistance in parallel with a
cell membrane capacitance and intracellular resistance. Nonlinear
weighted least squares curve-fitting applied to the multifrequency
impedance spectrum was used to extrapolate extracellular fluid
resistance and total fluid resistance at the low-and high-frequency
limits. Resistance data were then converted to fluid volume data through
a limb segment model [19].

Bioimpedance is a very sensitive measure of limb fluid volume, and
no accepted gold standard with better resolution exists with which to
compare it. However, comparison with lower-resolution volume measurement
techniques has shown it to correlate well with deuterium oxide and
bromide dilution techniques [7,20-23] as well as with magnetic resonance
imaging [24].

In this article, only extracellular fluid volume data are
presented. Extracellular fluid volume is likely the primary source of
fluid volume changes within the residual limb over short-term intervals
(<40 min). Intracellular fluid transport would be expected to be too
slow to accomplish significant volume change within the approximately 30
to 40 min long test sessions conducted here.

[FIGURE 1 OMITTED]

Because the electrodes were within the prosthetic socket during
testing, the standard alligator clips used to connect the XiTRON
Technologies instrument to the electrodes could not be used. A custom
four-pin Delrin connector designed to accommodate gold-plated pins (WPI
Viking, Cooper Interconnect; Chelsea, Massachusetts) was developed to
attach four 28-gauge insulated lead wires from the XiTRON Technologies
instrument cable to the electrodes. Each wire was strain-relieved and
mechanically stabilized at the electrode tab connection by looping of
the wire onto the tab, light soldering of the exposed end to the tab,
and then placement of a polystyrene disk (9 mm diameter, 0.25 mm
thickness) over the solder connection. The tab was then covered with a
single layer of vinyl electrical tape (Super 88, Scotch, 3M; St. Paul,
Minnesota). A 9 x 26 mm section was cut from the adhesive part of a Band
Aid (Johnson & Johnson; New Brunswick, New Jersey). The outer
surface of the Band Aid was glued (Skin-Bond Cement, Smith & Nephew;
St. Petersburg, Florida) onto the outside of the electrical tape so that
the Band-Aid's sticky surface was exposed. The Band-Aid's
sticky surface was put on the skin so that, like the electrode, it stuck
to the skin. This attachment method was necessary to avoid excessive
strain application to the tab-solder connection that otherwise would
have caused mechanical damage and failure of the electrode.

Protocol

After informed consent was obtained, subjects were asked not to
consume alcohol or caffeine on the day of testing before coming into the
laboratory. After arriving at the laboratory, the subject walked briefly
on a treadmill (Clubtrack, Quinton Instruments Company; Bothell,
Washington) to determine a normal self-selected walking speed. The
subject then sat quietly in a chair while basic subject information was
collected: age, date of amputation, cause of amputation, health status,
regular activities, recent limb health history, and recent changes to
prosthesis. Then, the subject removed his or her prosthesis, and the
research practitioner inspected the residual limb to ensure there were
no sores or injury. The skin locations where electrodes were to be
placed were cleaned by gentle rubbing with sandpaper (Red Dot[TM] Trace
Prep 2236, 3M). Sandpaper was used rather than an alcohol-based cleaning
agent because alcohol dries the skin and reduces conductance [18]. Two
current-conducting and two voltage-sensing strip electrodes (XiTRON
Technologies) (77 x 20 mm contact surface, 0.81 mm thickness) were
placed on the lateral posterior surface of the subject's limb such
that the long axes of the electrodes were parallel to each other and
perpendicular to the long axis of the residual limb (Figure 1). A thin
layer of coupling gel (Couplant D, Panametrics, General Electric
Company; Fairfield, Connecticut) was applied to the electrode before it
was placed on the skin. The proximal voltage-sensing electrode was at
the level of the patellar tendon, proximal of the fibular head. The
distal current-injecting electrode was positioned as far distally as
possible but still on the relatively cylindrical portion of the
residuum. The distal voltage-sensing electrode was positioned between
3.0 and 4.5 cm proximal to the distal current-injecting electrode. The
proximal current-injecting electrode was positioned on the thigh outside
the proximal socket brim but beneath the liner or suspension sleeve.
Lead wires were strain relieved on the posterior residual limb surface
with use of rectangular pieces (6.0 x 3.5 cm) of Tegaderm (3M). So that
vacuum within the socket was maintained, a piece of Tegaderm was also
placed over the lead wires at the proximal edge of the suspension
sleeve. As long as the Tegaderm was placed flat on the skin and over
each lead wire individually, air did not enter along the edges of the
lead wires into the socket. During each test session, the research
practitioner carefully inspected the socket to ensure vacuum was
well-maintained.

For subjects using a manual elevated vacuum system (Harmony, Otto
Bock; Minneapolis, Minnesota), the unit was modified so that the vacuum
could be easily connected or disconnected during the session. This
capability was achieved by placement of a stopcock in series in the
plastic tubing connecting the mechanical pump to the valve in the
socket. By adjusting the stopcock orientation, the research practitioner
could set either a vacuum or a suction condition. For the electronic
elevated vacuum systems, no such modification was necessary because they
were powered by an adjustable electronic pump. The vacuum level was
increased or decreased with switches on the vacuum unit.

Data collection involved periods of sitting, standing, and walking.
For the sitting portion of the protocol, the height on the
subject's chair was adjusted so as to maintain the knee in
100[degrees] to 140[degrees] of extension of extension with the foot
resting on the floor. During the standing portions of the protocol, the
subject stood on a 6.4 cm high platform with an electronic weight scale
(349KLX Health-O-Meter, Pelstar LLC; Alsip, Illinois) embedded within it
so that the top of the scale was flush with the surface. Weight bearing
on the limb instrumented with the bioimpedance electrodes was monitored
at a 2 Hz sampling rate and observed by one of the researchers on a
computer screen. If the weight on the scale deviated by more than 10
percent of half the subject's body weight, then the subject was
instructed by the research practitioner to shift his or her weight to
the appropriate leg to achieve more equal weight bearing. A 10 percent
threshold was used because in preliminary investigations we determined
that this range of weight-bearing change did not typically induce
changes in bioimpedance results and it did not typically necessitate
continual instruction to the subject for weight shifting. In preliminary
investigations, continual weight shifting caused some subjects to
stiffen up, distorting the data of interest. During the walking
segments, the subject walked on the treadmill at the speed established
at the outset of the session as described. Subjects walked for 3 or 5
min periods (depending on the protocol) on the treadmill. A 3 or 5 min
period was selected because all subjects could achieve those durations
without fatigue. Further, we desired to change socket vacuum conditions
during tests but not prolong the entire test to more than 40 min total.

Custom MATLAB code (v. 7.4, The MathWorks; Natick, Massachusetts)
was written so as to display the limb fluid volume data to the
researchers in essentially real time (3 s delay). This display was
essential during data collection to ensure subjects did not occlude a
major blood vessel during sitting (apparent as a rapid change in limb
fluid volume during sitting) and to ensure equipment was functioning
properly throughout the session. Clinicians on the research team found
the immediate data presentation helpful toward clinical interpretation.
Data from the electronic scale sampled at 2 Hz were collected
simultaneously with the bioimpedance data, using the same computer as
used to display bioimpedance data. We put time stamps in the code so
that bioimpedance data could be lined up with subject weight-bearing
data during standing.

Data Processing and Analysis

After the session was completed, software provided by the
manufacturer (version 2.2, XiTRON Technologies) was used to process the
collected bioimpedance data. The software used the Cole model approach
as described in the literature [17], optimizing a nonlinear least
squares error of magnitude and phase to determine extracellular fluid
resistance. To avoid deleting data points solely to force fit the Cole
model, data at a frequency was removed from analysis only if including
it decreased the total weighted least squares error with specified
limitations. The data were then converted to extracellular fluid volume
with use of a well-accepted model [19] and then expressed as a
percentage of the extracellular fluid volume measured at the end of the
initial 2 min sit interval (prosthesis donned).

Later during postprocessing, to facilitate inspection of the data,
a 10-point moving mean of the percent change in extracellular fluid
volume was plotted, shifted back 5 points to realign it with the
original data. Because the sampling rate (~1 Hz) was less than the
walking frequency, measuring fluid volume change within each step was
not possible. However, in this analysis, it was the change over the
course of the 3 or 5 min walking interval that was of interest; thus,
use of the moving mean curve in analysis was considered appropriate.

RESULTS

On the basis of previous studies testing the capabilities of
bioimpedance analysis for lower-limb assessment [25], we considered 0.2
percent to be the lowest percentage fluid volume change that could be
accurately resolved in this research. This resolution limit was
principally due to bit quantification error in the instrument and
processing algorithm. Thus, in the presentation below, differences in
limb fluid volume change between two test conditions on a subject were
considered meaningful only if they were greater than 0.2 percent.

This 42 yr old male had his left lower limb amputated 5 yr prior as
a result of traumatic injury. His limb was revised 16 mo after his
initial amputation because of complications from surgical staples within
his residuum. His residual limb length was 16 cm from the midpatellar
tendon to distal end and cylindrical in shape with a fair amount of
hair. He was 180 cm in height, 88 kg in mass, and a K-4 level [26]
ambulator. His prosthesis was a total contact socket design with an
elevated vacuum unit and a dynamic response foot (Renegade Freedom Foot,
Freedom Innovations; Irvine, California). He used elevated vacuum
because of prior diurnal residual limb volume change problems using a
Pelite liner and because he had an interest in trying elevated vacuum
suspension. One year before our bioimpedance testing, he switched from a
manual elevated vacuum system (Harmony, Otto Bock) to an electronic
system (eVAC, Smith Global; Laurie, Missouri). In bioimpedance testing,
he was tested with his electronic system. He had a history of blister
problems when he set the vacuum pressure at the highest setting, 15 psi
(104 kPa), instead of his usual setting, 10 psi (69 kPa). These pressure
settings were the values labeled on the elevated vacuum unit; we did not
measure the actual pressures. Thus, we do not know whether the stated
pressure levels were achieved in the present study. The subject
typically wore his prosthesis for 16 h/d and wore a 5-ply and 3-ply sock
outside of the liner. He did not add or remove socks during the day.

Protocol

Throughout the session, the vacuum was operated at the
subject's normal vacuum setting. After sitting comfortably for 2
min, the subject stood with equal weight bearing for 5 min and then
walked on the treadmill at his preferred walking speed for 5 min. After
sitting quietly for 2 min, he again stood for 5 min with equal weight
bearing and then walked on the treadmill for 5 min. He then sat, doffed
his prosthesis and liner, and sat quietly for 10 min. Thus, the total
session duration was approximately 34 min.

We attempted to conduct additional sessions with reduced vacuum
pressure settings during ambulation so as to evaluate the influence of
vacuum pressure intensity on the bioimpedance measured. However, the
subject was unable to walk comfortably under those conditions and had
trouble maintaining adequate suspension.

Results

Results from this subject showed that limb fluid volume decreased
during stands and increased during walks (Figure 2). The decreases
during stands averaged 0.9 percent and the increases during walks
averaged 1.5 percent. Thus, the increase from walking more than offset
the decrease from the immediately prior standing (1.5% > 0.9%). The
cyclic changes during the walks visible in the plot (Figure 2) were due
to pistoning and/or deformation of the residual limb within the socket,
and they are typical of reports in prior investigations [25,27]. Fluid
volume gradually increased over the course of the session (beginning of
1st stand to end of 2nd walk) of 2.1 percent. The peak-to-peak change in
fluid volume during walking averaged 2.0 percent. The fluid volume
change during the 10 min period during sitting after doffing was 1.7
percent.

These subjects were short-term users (3^4 wk) of the manual
(Harmony) system prior to bioimpedance testing. For each of the three
subjects, a new socket was made to use with the elevated vacuum
prosthesis. No locking pin was used with the elevated vacuum socket. All
had residual limbs with prominent bony structures and little redundant
soft tissue.

Case 2

This male subject was 61 yr of age and had his amputation 5 yr
prior as a result of traumatic injury. His 17 cm long residual limb was
conical in shape with a fair amount of hair and little soft tissue. He
was 175 cm tall, 73 kg in mass, and a K-4 level ambulator. He was
healthy and did not take any medications, though he used to be a smoker
(for 20 yr; stopped smoking more than 5 yr ago) and his blood pressure
was on the high end of normal. His regular prosthesis was a patellar
tendon bearing (PTB) endoskeletal prosthesis with a gel liner and
lock-and-pin suspension, and his prosthesis was equipped with a dynamic
response foot (Luxon Max, Otto Bock). He wore his prosthesis for at
least 16 h/d. He typically did not add socks to compensate for limb
volume change except on days when he was very active. He was a good
candidate for elevated vacuum because of his high level of activity,
desire for optimal proprioception, and need for fewer sock changes to
eliminate pistoning during active days. Before bioimpedance testing, he
was fitted with a new socket equipped with a P2 Harmony vacuum unit and
a Seattle Lightfoot (TruLife; Dublin, Ireland). He used this prosthesis
exclusively for the 3 wk before bioimpedance testing.

Case 3

This subject was a 48 yr old male and had his amputation 24 yr
prior as a result of traumatic injury. His 19 cm long residual limb was
conical in shape with a fair amount of hair and very little soft tissue.
He was 188 cm in height and 80 kg in mass. A K-4 level ambulator, this
subject was healthy, did not take medications, and used his prosthesis
for at least 12 h/d. He regularly used a PTB endoskeletal prosthesis, a
flexible socket liner with a gel liner, a lock-and-pin suspension, and a
Freedom Innovations dynamic response prosthetic foot. He had a history
of ingrown hairs/blisters that formed when his prosthesis pistoned
excessively as a result of limb volume changes. His limb typically
changed volume after high activity. He was a good candidate for elevated
vacuum because of his high activity, mechanical aptitude, need for
superior suspension, and desire to not add socks throughout the day to
compensate for volume fluctuations. Before bioimpedance testing, he was
fitted with a new socket equipped with a P2 Harmony vacuum unit and a
Seattle Lightfoot. He used this prosthesis exclusively for the 3 wk
before bioimpedance testing.

Case 4

This subject was a 54 yr old male who had a transtibial amputation
4 yr prior as a result of traumatic injury. His residual limb was 23 cm
in length, conically shaped, with good hair and sensation. He had a
distal neuroma removed 4 mo before bioimpedance testing. He was 188 cm
in height, 77 kg in mass, and a K-3 level ambulator. He used his
prosthesis for approximately 16 h/d. He had a history of smoking and
high cholesterol and had been diagnosed with peripheral arterial disease
and peripheral vascular disease. He had pain in his calf when ambulating
more than two blocks. Because of volume fluctuation problems in his
limb, he typically added socks during the day. In an effort to enhance
suspension and reduce volume fluctuation problems, 4 wk before
bioimpedance testing, his regular prosthetist switched him from a PTB
endoskeletal prosthesis with a gel sock, Pelite liner, and neoprene
suspension to a manual elevated vacuum system (P2 Harmony). He used this
manual elevated vacuum socket during bioimpedance testing, with a
dynamic response Flex-Foot (Ossur Americas; Foothill Ranch, California).

Protocol

For these subjects, part of the trial was conducted with the vacuum
off (suction) and part with it on (elevated vacuum). The protocol was
started with the stopcock set for a suction socket. After 2 min of
sitting, the subject stood with equal weight bearing for 3 min. He then
walked on the treadmill for 3 min at his nominal walking speed. The
subject then stopped, and the stopcock was turned so as to apply
elevated vacuum to the prosthesis. The subject then walked on the
treadmill again for 3 min. After 2 min of sitting, the subject stood and
walked for 3 min each, still with the elevated vacuum. The stopcock was
then switched back to a suction socket, and the subject again walked for
3 min. Walking intervals of 3 min duration were used in these studies
rather than 5 min because of concern that subjects would experience
discomfort using the heavy Harmony system without elevated vacuum. We
were concerned that subjects would not be able to complete 5 min
treadmill walking with suction alone. Also, using this ordering (walk
without elevated vacuum, walk with elevated vacuum, sit, walk with
elevated vacuum, and walk without elevated vacuum), we would be able to
distinguish a continuous increase or decrease in limb fluid volume over
the session from influence of elevated vacuum. For cases 2 and 3,
instrumentation problems occurred during the last walk; thus, data from
only the first two walks were included in the analysis described below.

Results

None of the three subjects demonstrated the continuous rise in limb
fluid volume during walking that case 1 did. Fluid volume tended to
increase and then plateau during walks when the vacuum was activated for
cases 2 and 3 (Figure 3). For case 2, limb fluid volume increased 1.2
percent during walking with the vacuum activated while for case 3, it
increased 0.4 percent (Table 1, column 3). For case 4, limb fluid volume
decreased 0.5 percent during the first walk with vacuum activated and
0.7 percent during the second walk with the vacuum activated, for an
average decrease of 0.6 percent.

[FIGURE 3 OMITTED]

Limb fluid volume after walking 3 min with suction was comparable
with that after walking 3 min with elevated vacuum for cases 2 and 3
(Table 1, column 4). The differences were 0.0 percent for case 2 and 0.1
percent for case 3. For case 4, limb fluid volume after 3 min of walking
with elevated vacuum averaged 0.7 percent less than that after 3 min
walking with suction. When vacuum was then reduced, fluid volume
increased 0.5 percent.

For all three (cases 2, 3, and 4), peak-to-peak fluid volumes were
less with elevated vacuum than with suction (Table 1, column 2). The
differences between elevated vacuum and suction were 2.3 percent for
case 2 (4.3% - 2.0%), 0.4 percent for case 3 (1.3% - 0.9%), and 0.7
percent for case 4 (3.6% - 2.9%).

Cases 5 and 6 both used electronic vacuum systems for short-term
intervals, approximately 4 wk. However, case 6 used the electronic
vacuum unit regularly outside the lab, while case 5 used it
intermittently. Both subjects had excessive redundant soft tissue in
their residual limbs, particularly distally.

Case 5

This subject was a 25 yr old female. She had her amputation 3 yr
prior as a result of traumatic injury and then had a surgical revision 2
yr later to remove excessive redundant soft tissue. Her residual limb
was 14 cm in length and was fleshy and bulbous with adherent tissue on
the distal tibia. She had a history of inflamed fungus from gel liner
use. She was 58 kg in mass, 160 cm in height, and a very active K-4
level ambulator, using her prosthesis for 16 h/d. She was a marathon
racer and triathlete and underwent much residual limb volume reduction
during long runs. Otherwise, her limb volume was stable. She regularly
used a PTB endoskeletal prosthesis with a gel liner and lock-and-pin
suspension and Renegade Ultralite Foot (Freedom Innovations). During
high activity she used an Iceflex endurance sleeve (Ossur) for auxiliary
suspension. She was fitted with a new socket equipped with an electronic
elevated vacuum system (e-Pulse, Otto Bock) and with a Seattle Foot by
her regular prosthetist for this investigation but did not feel
comfortable wearing it regularly because she was concerned about falling
while at her waitressing job. Thus, she used the elevated vacuum system
intermittently. She was tested with her regular prosthesis approximately
10 wk prior to testing with the elevated vacuum system.

Case 6

This subject was a 34 yr old male and had his limb amputation as a
result of traumatic injury 3 yr prior. His residual limb was 19 cm in
length and was bulbous with redundant soft tissue and a prominent distal
tibia. His residual limb had much hair and was very sensitive to pain.
He was 102 kg in mass, 188 cm in height, and a K-3 level ambulator. He
wore his prosthesis for approximately 15 h/d. He regularly used a PTB
endoskeletal prosthesis with a gel liner and lock-and-pin suspension and
a dynamic response foot (Freedom Innovations). During moderate or high
physical activity, he added socks to accommodate residual limb volume
reduction. He was tested first using his regular prosthesis. Then for
this investigation, he was fitted by his regular prosthetist with a new
socket equipped with an electronic elevated vacuum system (e-Pulse) with
a Seattle LightFoot. He used that system exclusively for 3 wk before
bioimpedance testing.

Protocol

After the electrodes were applied, the subject sat quietly for 2
min. Each subject walked at a selected vacuum setting for 3 min, first
with the setting increased from one 3 min walk to the next. Four
settings were possible on the electronic elevated vacuum unit, with
"4" being the highest pressure (labeled as "60 kPa"
by the manufacturer). The other settings were "1" (25 kPa),
"2" (36 kPa), and "3" (48 kPa). We did not measure
vacuum pressures during the studies; thus, the actual pressures achieved
and their fluctuations during use were unknown. Pressure fluctuations
during walking have been reported for other products [28]. The subject
then sat quietly for 2 min and then walked again for 3 min at the same
vacuum settings as before sitting. Then, the vacuum setting was
decreased from one walk to the next. For case 5, two settings were used:
the 1-setting and 4-setting (highest vacuum pressure). For case 6, four
settings were used: the 1-setting, 2-setting, 3-setting, and 4-setting.
Case 5 preferred the 4-setting, while case 6 preferred the 3-setting.
Because the vacuum pump could be heard activating early on during each 3
min walk, we concluded that the elevated vacuum pressures during walks
with the high settings were not maintained during subsequent walks at
lower vacuum settings. In other words, the result that changes in limb
fluid volume were small when vacuum pressure settings were reduced
(Figure 4) was not a result of high vacuum pressure being maintained
from the previous 3 min walk interval.

Results

When the vacuum pressure was increased from the 1-setting to the
4-setting, peak-to-peak limb fluid volume changes did not change as much
as they did for cases 2, 3, and 4 when the socket was changed from
suction to manual elevated vacuum. Peak-to-peak differences for the
1-setting versus the 4-setting averaged 0.2 percent for case 5 (3.4% -
3.2%) and 0.2 percent for case 6 (2.0% 1.8%).

Limb fluid volume increased more during the walks at the higher
vacuum pressure setting than at the lower vacuum pressure setting. The
difference in fluid volume increase during walks for the 4-setting
compared with the 1-setting was 0.3 percent for case 5 (0.5% - 0.2%) and
0.4 percent for case 6 (1.0% - 0.6%).

Limb fluid volume was greater at the end of walks at the 4-setting
than at the 1-setting for both subjects (Table 1, column 4). The
increase in fluid volume upon elevating the vacuum setting was greater
than the decrease in fluid volume upon reducing the vacuum setting. For
case 5, the fluid volume change from elevating the vacuum setting was
0.6 percent and the change for reducing the setting was -0.2 percent.
For case 6, the fluid volume change from elevating the vacuum setting
(difference between 1-setting and 4-setting) was 1.9 percent and the
change for reducing the vacuum setting was -1.0 percent. However, note
that for case 6, four vacuum settings were tested, unlike case 5, for
whom only two settings were tested. This difference in protocol meant
that case 6 walked longer using elevated vacuum, which might partially
explain why case 6's fluid volume changes were higher than case
5's.

For case 6, increasing the vacuum setting from 1 to 2 to 3 to 4
showed a gradual increase in limb fluid volume, but decreasing the
setting from 4 to 3 to 2 to 1 did not show a gradual fluid volume
decrease (Figure 4, right panel). Instead, a sudden decrease only was
seen for changing from the 2-setting to the 1-setting. No change in
peak-to-peak fluid volume was observed for any pair of settings (walks
at the same vacuum pressure setting) for either subject. From visual
inspection of the fluid volume changes over time for both subjects
(Figure 4), the limb fluid volume changes induced from changing the
vacuum pressure appear to be superposed on a gradual limb fluid volume
increase during the session.

For cases 5 and 6 (described above), as well as case 7 (described
below), we compared each subject's results using an elevated vacuum
socket to results using a lock-and-pin socket (different socket). The
tests were conducted on different days. Data collection sessions for
each subject were at approximately the same time of day for cases 5 and
6 but were at a different time of day for case 7 (Table 2, column 1).
Vacuum settings on the units for cases 5 and 6 were the 3-setting and
4-setting, respectively.

Case 7

This subject was a 61 yr old male with type 2 diabetes. As a result
of an unhealed neuropathic foot ulcer, he had a right transtibial
amputation 8 yr prior. He was 163 cm tall, 99.5 kg in mass, and a K-2
level ambulator, using his prosthesis at least 12 h/d. He regularly used
a PTB endoskeletal prosthesis with a gel liner and lock-and-pin and a
Fusion Foot (Ohio Willow Wood; Mt. Sterling, Ohio). His skin was thin
and fragile with poor sensation, and in the past, he had had blisters
and wounds on his residual limb. He did not typically add socks during
the day to accommodate residual limb volume reduction, though the
research practitioner considered this practice a reflection of his
neuropathy. She believed that he did not sense his limb volume reduction
and the need to add a sock. He underwent bioimpedance testing with his
regular prosthesis first. He was then fitted with a new socket equipped
with a P2 Harmony unit and Seattle Lightfoot.

[FIGURE 4 OMITTED]

He used this prosthesis exclusively for 3 wk before bioimpedance
testing.

Protocol

The protocol for the session using the lock-and-pin suspension and
the session using elevated vacuum was the same and was similar to that
described for case 1. After the electrodes were put on the residual limb
and the prosthesis donned, the subject sat comfortably in a chair for 2
min. Then, the subject stood with equal weight bearing for 5 min and
subsequently walked on the treadmill for 5 min. After sitting quietly
for 2 min, the subject again stood for 5 min with equal weight bearing
and then walked on the treadmill for 5 min. The subject then sat, doffed
the prosthesis and liner, and sat quietly for 10 min. The fluid volume
change after doffing was calculated as the fluid volume after 10 min of
sitting minus that measured immediately after the prosthesis and liner
were removed. The fluid volume during standing right after the last walk
minus the fluid volume at the outset of the first stand represented the
session fluid volume change.

Results

The results showed subject-dependent trends. Peak-to-peak residual
limb fluid volume for elevated vacuum sockets compared with lock-and-pin
sockets was higher for case 6, lower for case 7, and not different for
case 5 (Table 2, column 2). Fluid volume changes during 5 min walks were
greater for elevated vacuum than for lock-and-pin for case 5 and not
different for cases 6 and 7 (Table 2, column 3). Fluid volume changes
during 10 min after doffing for elevated vacuum were lower than for
lock-and-pin for case 6 and higher for cases 5 and 7 (Table 2, column
4). For all three cases, the difference in fluid volume from the brief
stand right after the last walk minus the fluid volume at the outset of
the first stand was more positive for elevated vacuum than for
lock-and-pin (Table 2, column 5). Thus, during the test session, subject
limbs tended to increase more (or decrease less) for elevated vacuum
compared with lock-and-pin.

Bioimpedance analysis has been validated against other techniques
for fluid volume assessment. In whole body analysis, bromide dilution
and bioimpedance extracellular fluid volume measurements were shown to
be highly correlated (r > 0.9) [20]. Limb segment muscle volumes
determined by magnetic resonance imaging were also shown to be highly
correlated to bioimpedance measurements (r > 0.9) [24]. Our
measurements of limb fluid volume change after doffing in the present
study (median of 2.0%) were less than those reported by Zachariah et al.
using an optical scanning method on six subjects (median of 5.0%) [29].
However, in Zachariah et al.'s study [29], measurements after
doffing were taken during standing, which would be expected to increase
volume changes compared with sitting, the postdoffing measurement
position in the present investigation. Because bioimpedance measurements
may be sensitive to movement of the voltage-sensing electrodes relative
to each other, we took precautions here to avoid electrode movement
detrimentally affecting interpretation of bioimpedance results.
Differences in limb fluid volume were calculated only for like postures,
e.g., during a walking interval, over a sitting interval (after
doffing), from standing with equal weight bearing at one point in time
to standing with equal weight bearing at another point in time. We
assumed that like postures had similar electrode positions relative to
each other.

Did Limb Fluid Volume Maintain or Increase During Walking when
Subjects Used Elevated Vacuum?

Residual limb fluid volume increased during short-term walks on all
healthy subjects (1, 2, 3, 5, 6) when elevated vacuum was used. However,
limb fluid volume also typically increased during short-term walks when
the vacuum was off or a lock-and-pin suspension was used instead. In
other studies, we have similarly found that healthy subjects experienced
limb fluid volume increases during short-term walks [27]. Our
interpretation of these findings is that the increase in fluid volume
during short-term walks is primarily a result of a rise in arterial
blood pressure and arterial dilation, resulting in more blood flow in
the residual limb and an increase in arterial to interstitial fluid
transport. During short-term walks, these physiological changes may be
more dominant than elevated vacuum toward increasing limb fluid volume.
Limb fluid volume decreases during stands immediately before walks might
have accentuated walking limb fluid volume increases in that they
temporarily dehydrated the residual limb. An interesting and needed area
of future investigation is bioimpedance monitoring of long-term walking
to determine whether elevated vacuum has a more dominant role in that
time frame.

The two subjects with compromised health, cases 4 and 7, were the
only subjects to demonstrate constant or decreasing residual limb
volumes during walks, consistent with results from previous case studies
[27]. Case 4 had peripheral arterial disease/peripheral vascular
disease, and case 7 was diabetic with poor sensation in his residual
limb. Ko'itka et al. [30], McLellan et al. [31], and Fromy et al.
[32] demonstrated that people with diabetes and sensory neuropathy
tended to have a reduced capability for pressure-induced vasodilation
compared with nondiseased subjects. Thus, for case 7, a reduced arterial
volume flow rate induced by a lack of pressure-injured vasodilation and,
for case 4, reduced arterial flow resulting from arterial disease may
explain why these subjects' limb fluid volume changes during walks
were lower than those of healthy subjects.

Only one subject in the present study (case 1) was a regular
long-term (>6 mo) user of elevated vacuum. His change in residual
limb fluid volume over the session (+2.1%) was much larger than that of
other subjects. Further studies need to be conducted to determine
whether this trend represents an adaptive response to elevated vacuum
over long-term use.

The single subject who demonstrated a reduction in limb fluid
volume during walks with both vacuum and suction sockets (case 4) was
unusual in that his socket, designed to accommodate a neuroma at the
anterior distal end of his residual limb, was wedge-shaped (conical) but
with a localized relief distally. This socket design may have pushed the
subject deeper into the socket when walking, and the wedging effect may
have reduced limb fluid volume. This socket design is contrary to that
suggested by companies marketing elevated vacuum products. Manufacturers
recommend total contact with uniform pressure distally. A systematic
study investigating the effects of socket shape on limb volume changes
during elevated vacuum use is warranted and would aid understanding of
how sensitive limb volume shifts are to socket shape.

For some, but not all, subjects, limb fluid volume increases during
walks were greater with greater vacuum, consistent with expectation. The
vacuum pulled soft tissues outward, helping to pull fluid into the
interstitial space within the residual limb, which increased or
maintained limb fluid volume. For no subject was limb fluid volume
change during walks lower with greater vacuum. One possible reason why
some of the subjects did not display limb fluid volume increases as
large as others may have to do with the size difference between the
socket and residual limb. If a socket were tight on the residual limb at
the outset of testing, then the socket would restrict residual limb
enlargement upon vacuum application. No or minimal limb volume increase
would occur. A loose socket at the outset of the testing protocol,
however, would allow the residual limb to enlarge when vacuum was
applied. This expectation is consistent with Goswami et al.'s
findings that limb volume increase was greater with elevated vacuum when
an oversized socket was used compared with a normal or an undersized
socket [4]. Thus, with a loose socket at the outset, a subject using
elevated vacuum would be expected to experience limb fluid volume
increase during short-term walks, but with a tight socket, no limb fluid
volume increase would be expected.

Did Limb Fluid Volume Increase when Vacuum was Turned from Off to
On or Vacuum was Increased from Low to High Pressure Setting?

Results depended on the type of elevated vacuum system used. For
the two electronic elevated vacuum users (cases 5 and 6), limb fluid
volume increased substantially after increasing vacuum pressure. The
increases from the 1-setting to the 4-setting on the vacuum device were
0.6 percent for case 5 and 1.9 percent for case 6. For the Harmony
users, however, limb fluid volume increases when switching from suction
to vacuum were low: 0.0, 0.1, and -0.7 percent (cases 2, 3, and 4,
respectively).

While the heavy weight of the Harmony system compared with the
electronic system probably contributed to these differences, we suspect
that differences in residual limb soft tissue content between the two
sets of subjects was also a dominant factor. The electronic elevated
vacuum users (cases 5 and 6) had very fleshy limbs with much redundant
soft tissue, unlike the Harmony users (cases 2, 3, and 4), who had bony
residual limbs. More soft tissue may have enhanced the capability for
fluid volume change.

The magnitudes of limb fluid volume change over the course of the
session for manual elevated vacuum in the present study were much lower
than limb volume changes reported by Board et al. [3]. Changes in the
present investigation ranged from -1.6 to +1.2 percent, and in Board et
al.'s study they ranged from -1.6 to +8.5 percent. Part of the
reason for the inconsistency is that different measurements were taken.
Limb fluid volume change was assessed in the present study, while Board
et al. assessed total limb volume change. Further, we measured in-socket
changes, while Board et al. measured out-of-socket changes. Board et al.
measured external limb shape before donning and after doffing by using a
casting and water displacement method, and we used bioimpedance while
the residual limb was within the socket. Further, the section of the
limb we tested did not include the distal end, unlike Board et al. who
included the entire residual limb. Our walking times totaled
approximately 10 min over the 30 min test session, while Board et
al.'s subjects walked continuously for 30 min and thus were more
physically exerted. One of our subjects (case 7) had his amputation for
dysvascular reasons, while all Board et al.'s subjects were
traumatic injury or congenital amputees. Thus, numerous variables might
have contributed to the substantial measurement differences between
Board et al.'s and the present study.

In the present study, results from electronic elevated vacuum users
showed that limb fluid volume increases from increasing vacuum pressure
were of greater magnitude than limb fluid volume decreases from
decreasing vacuum pressure (cases 5 and 6). This result suggests that
resistance toward driving fluid out of the residual limb was greater
than for bringing fluid into the limb. It is unclear whether this
phenomenon was a result of using elevated vacuum, reflected the prior
activity history, or was a physiological characteristic of these
particular subjects. In terms of prescribing and adjusting vacuum
pressure on individual users of elevated vacuum, these differences in
fluid transport resistance are important to understand. Further
investigation is needed to understand how elevated vacuum affects
physiological fluid transport.

Did Cyclic (Peak-to-Peak) Fluid Volumes During Walking Change when
Vacuum was Activated?

While the sampling rate of our system was lower than the walking
rate of the subjects, we considered comparing peak-to-peak fluid volumes
for different test conditions acceptable because none of the subjects
had a walking rate that was a multiple of the sampling rate (1 Hz). If
the walking speed were a multiple of the instrument's sampling
rate, then an aliasing problem with a consistent error in peak-to-peak
limb fluid volume would occur, invalidating the peak-to-peak assessment.

The result in the present study that the peak-to-peak fluid volume
decreased when subjects switched from suction to elevated vacuum
(Harmony system) is consistent with expectation. Limb fluid volume
changes within a step decreased with higher vacuum, presumably because
there was less pistoning. Also possible is that the changes in
peak-to-peak fluid volume reflect changes in muscle activation. Possibly
subjects felt that the prosthetic socket was looser on the residual limb
when elevated vacuum was off compared with on and, as a result,
contracted their musculature more forcefully, inducing greater fluid
volume change. Electromyography assessment would help evaluate this
hypothesis.

Peak-to-peak fluid volumes did not decrease when the electronic
elevated vacuum (e-Pulse system) was used and the vacuum was increased
from the 1-setting to the 4-setting. This difference in result between
the electronic and manual vacuum systems may have occurred because the
change in vacuum pressure for the electronic system (1-setting to
4-setting) was likely less than the change in vacuum pressure for
suction versus manual vacuum (Harmony system). It is also feasible that
a relatively low threshold vacuum pressure was sufficient to
substantially reduce pistoning in the electronic elevated vacuum
sockets, and the 1-setting on the system (25 kPa according to
manufacturer literature) was above this threshold. Note, however, that
vacuum pressure was not measured in the present study; thus, actual
pressures are unknown. Other possible explanations include that the
manual system was much heavier than the electronic vacuum system,
inducing greater pistoning and thus greater peak-to-peak limb fluid
volume change when the vacuum pressure was increased; the electronic
system applied continuous vacuum, unlike the manual system, where vacuum
was applied intermittently (i.e., only during walking); and the
electronic elevated vacuum users (cases 5 and 6) had much redundant soft
tissue, unlike the Harmony users (cases 2, 3, and 4), who had relatively
bony residual limbs. Too many variables exist to allow us to determine
what caused differences in peak-to-peak walking fluid volume dependence
on vacuum pressure for electronic versus manual systems. In the future,
all the just-listed variables will need to be considered in studies
designed to investigate peak-to-peak limb fluid volume changes during
walking.

All three subjects (cases 5, 6, 7) demonstrated more positive
changes in limb fluid volume from the beginning to the end of the test
session when using elevated vacuum compared with a lock-and-pin
suspension socket. This result is consistent with Board et al. [3] and
Gerschutz et al. [5]. Both researcher groups found greater volume
increase from the beginning to the end of the session for elevated
vacuum versus suction, though they measured out-of-socket volumes as
opposed to in-socket volumes as assessed here. Unlike Gerschutz et
al.'s results from a single subject [5], though, for two of the
three subjects tested here during 10 min after doffing (cases 5 and 7),
limb fluid volume increases were greater for elevated vacuum than for
nonelevated vacuum. However, the present study was designed differently
than Gerschutz et al.'s. In the present study, two different
sockets were used, one with elevated vacuum and one with lock-and-pin.
The two sockets were not necessarily of the same volume. In Gerschutz et
al.'s study, one socket, a consistent volume, was used at different
vacuum settings (elevated vacuum, suction). These differences in study
design might explain differences in the results. The small numbers of
subjects further limits interpretation. The meaning of postdoffing limb
volume change toward in-socket volume change and subject well-being is a
topic in need of further investigation.

The magnitude of limb fluid volume change over the session may
depend on the health of the subject, similar to limb fluid volume
changes during walking. In similar reports, we have noted that limb
fluid volume changes over the session reflect subject health. In the
present study, we expect that the reduced fluid volume change over the
session for case 7 is almost certainly due to his poorer health status
(diabetic, neuropathy) compared with cases 5 and 6. However, it is
noteworthy that this subject still demonstrated an effect of elevated
vacuum. His fluid volume loss over the session using elevated vacuum was
less than that using suction (Table 2, column 5), which is an
encouraging result toward potential use of elevated vacuum on vascularly
compromised subjects. However, much further testing is needed before
clinical practice recommendations can be made.

Peak-to-peak fluid volume changes were not consistently larger when
a lock-and-pin socket was used compared with elevated vacuum. However,
for the subject showing trends opposite to those expected (case 7)
(Table 2, column 2), larger peak-to-peak fluid volumes using vacuum than
lock-and-pin, the two test sessions were conducted at different times of
the day. For the lock-and-pin test day, data were collected in the
morning, while for the elevated vacuum test day, data were collected in
the afternoon. This was unlike cases 5 and 6, who had afternoon test
days for both sessions. Experience testing other subjects has shown limb
fluid volume changes over a session depend on the time of day of testing
[33]. Morning session and afternoon session results were typically
different. If, in the present study, case 7's residual limb was
smaller in the afternoon and no accommodation was performed, then
greater pistoning and thus greater peak-to-peak fluid volume changes
would be expected. Also possible is that differences in results between
subjects for elevated vacuum versus lock-and-pin are a result of
different socket designs. Further investigation is needed to distinguish
the influence of elevated vacuum from the influence of time of day and
of socket-to-limb size on residual limb fluid volume change.

Future Research

The results from this series of case studies do not consistently
demonstrate that elevated vacuum maintained or increased limb fluid
volume nor do they consistently demonstrate that elevated vacuum had no
effect. Instead, elevated vacuum maintained or increased limb fluid
volume on six of the seven subjects and affected some measures of limb
fluid volume change but not others.

Results from these cases suggest that in future research efforts
evaluating elevated vacuum, researchers need to consider a number of
study design variables that may influence limb volume change
measurements. These variables need to be considered when test results
between two different conditions (e.g., elevated vacuum vs suction) are
compared. Variables include--

1. Time of day of test.

2. Size of residual limb relative to size of socket.

3. Use of elevated vacuum as the regular prosthesis (i.e., subject
accommodation).

4. Subject health.

5. Time into session that measurements are made and ordering of
interventions within a session.

6. Limb soft tissue mechanical consistency.

7. Socket shape.

8. Weight differences between prostheses tested.

9. Time after doffing that measurements are taken (if out-of-socket
measurement technique used).

Research efforts directed toward identifying which individuals are
good candidates for elevated vacuum and why will facilitate effective
application of elevated vacuum technology to appropriate patients. Also
helpful would be studies that facilitate the design of computer
algorithms within elevated vacuum units to appropriately regulate the
magnitude of vacuum pressure to maintain limb volume and good suspension
without subjecting residual limb soft tissues to undue risk.

CONCLUSIONS

This series of case studies on seven subjects showed that some
subjects demonstrated less decrease (or more increase) in limb fluid
volume using sockets with elevated vacuum compared with suction sockets
or lock-and-pin suspension sockets, while others did not. Some measures
of limb fluid volume changed consistently, while others did not. A
number of variables may affect limb fluid volume change. When designing
future research studies, investigators need to consider these variables
in study design, particularly when comparing elevated vacuum to another
socket design.

at a Glance

[ILLUSTRATION OMITTED]

We measured changes in fluid volume inside the residual limbs of
seven people with transtibial amputations while they walked with their
prosthetic limbs. Bioimpedance analysis, a new technique in the
prosthetics field, calculated limb fluid volume change. Results showed
that, in general, fluid volume decreased less or increased more when the
participants used elevated vacuum sockets than when they used low vacuum
or suction sockets. Time of day, soft tissue, socket size, type of
socket normally worn, and health might also affect fluid volume changes
inside residual limbs.

Financial Disclosures: The authors have declared that no competing
interests exist.

Funding/Support: This material was based on work supported by the
National Institutes of Health (grant NIH R01HD060585).

Additional Contributions: Clinical assistance from Ryan Blanck,
CPO, of the Northwest Prosthetic and Orthotic Clinic and the Seattle
Department of Veterans Affairs prosthetics center is appreciated.
Timothy R. Myers is now with Newberg Roadhouse LLC.

Institutional Review: Internal review board approval from the
University of Washington Human Subjects Division was obtained before
testing on any subjects, and informed consent was obtained before any
study procedures were initiated.

Participant Follow-Up: The authors do not plan to inform
participants of the publication of this study because contact
information is unavailable.

Submitted for publication November 17, 2010. Accepted in revised
form April 27, 2011.